The invention relates to a gas turbine engine, particularly to an integrated duct diffuser assembly for directing outward flow of compressed air from a centrifugal compressor impeller to an axial rearward diffused annular flow.
The compressor section of a gas turbine engine includes a diffuser downstream of the compressor turbine and a centrifugal impeller upstream of the combustor. The function of a diffuser is to reduce the velocity of the compressed air and simultaneously increase the static pressure thereby preparing the air for entry into the combustor at a low velocity. High pressure low velocity air presented to the combustor section is essential for proper fuel mixing and efficient combustion.
The present invention is particularly applicable to gas turbine engines which include a centrifugal impeller at the high pressure stage of the compressor. Impellers are used generally in smaller gas turbine engines. A compressor section may include axial or mixed flow compressor stages with the centrifugal impeller as the high pressure section, or alternatively a low pressure impeller and the high pressure impeller may be joined in series.
A centrifugal compressor impeller draws air axially from a small diameter. Rotation of the impeller increases the velocity of the air flow as the input air is directed over impeller vents to flow in a radially outward direction under centrifugal force. In order to redirect the radial flow of air exiting the impeller to an annular axial flow for presentation to the combustor, the diffuser assembly is also provided to redirect the air from radial to axial flow and to reduce the velocity and increase static pressure.
A conventional diffuser assembly generally comprises a machined ring which surrounds the periphery of the impeller for capturing the radial flow of air and redirecting it through generally tangential orifices into an array of diffuser pipes. The diffuser pipes are generally brazed or mechanically connected to the ring and have an increasing cross-section rearwardly.
Fabrication of the diffuser pipes is extremely complex since they have a flared internal pathway that curves from a generally radial tangential direction to an axial rearward direction. Each pipe must be manufactured to close tolerances individually and then assembled to the machined diffuser ring. Complex tooling and labour intensive manufacturing procedures result in a relatively high cost for preparation of the diffusers.
In operation as well, diffusers often cause problems resulting from the vibration of the individual diffuser tubes. To remedy vibration difficulties, the diffuser pipes may be joined together or may be balanced during maintenance procedures.
From an aerodynamic standpoint the joining of individual diffuser pipes to the machined ring results in surface transitions which detrimentally affects the efficiency of the engine. On the interior of the pipe as it joins the orifice in the ring, there is often a step or transition caused by manufacturing tolerances in the assembly and brazing procedures. Since the air in this section flows at extremely high velocity, the disturbance in air flow and increase in drag as the air flows over inaccurately fit transitions can result in very high losses in efficiency.
In general, the conventional design of diffusers is not optimal since their complex structure requires a compromise between the desired aerodynamic properties and the practical limits of manufacturing procedures. For example, the orifices in the diffuser ring are limited in shape to cylindrical bores or conical bores due to the limits of economical drilling procedures. The shape of the diffuser pipes themselves is also limited by the practical considerations of forming their complex geometry. In general, the diffuser pipes are made in a conical shape and bent to their helical final shape prior to brazing. Whether or not this conical configuration is optimal for aerodynamic efficiency becomes secondary to the considerations of economical manufacturing.
In order to reduce the tooling and manufacturing costs associated with prior art diffuser assemblies and optimize the diffuser structure for improved aerodynamic efficiency and vibration behavior without concern for the manner in which the diffuser will be actually manufactured, an improved diffuser design is described by Brand et al, in their U.S. patent application Ser. No. 09/233,023, filed on Jan. 20, 1999 commonly owned, which is herein incorporated by reference. The improved diffuser design of Bland et al is simply constructed of two concentric nested shells, secured together by brazing, each shell having opposing mating grooves which when the shells are nested together, define an array of diffuser ducts extending from an inner peripheral compressor impeller casing to an annular axially directed outer edge.
The diffuser design described by Brand et al significantly reduces the tooling and manufacturing costs associated with prior art diffuser assemblies because the individual pipes are replaced by the array of diffuser ducts defined between two concentric nested shells. Nevertheless, the mating of the opposing grooves on the respective nested shells still requires relatively accurate tooling and manufacturing, and therefore it is desirable to further improve the design of the diffuser assembly to better achieve the aims for which the diffuser assembly described by Brand et al is intended.
It is an object of the invention to provide a diffuser assembly which significantly reduces the tooling and manufacturing costs associated with prior art diffuser assemblies.
It is another object of the invention to provide a diffuser assembly which provides greater flexibility to the designers of gas turbine engines enabling them to optimize the diffuser structure for improved aerodynamic efficiency and vibration behavior without concern for the manner in which the diffuser will be actually manufactured.
It is a further object of the invention to provide a diffuser assembly which has shorter development time for new engines and considerably shorter lead time in normal production by minimizing the operations required for production.
It is a further object of the invention to eliminate the internal transversal steps between the diffuser pipes and a separate internal machined ring of the prior art.
It is a further object of the invention to lower the weight of engines by reducing the number of parts in a diffuser assembly, and using curved or variable diffuser ducts to reduce the gas generator case diameter.
The invention provides a diffuser assembly for directing a flow of compressed air with a radial component from a centrifugal compressor impeller to a diffused annular flow having an axial component. The diffuser assembly comprises a first bowl-shaped casing shell having a first annular diffuser portion, a first downstream annular edge co-axial with the first annular diffuser portion, and a surface having a plurality of grooves extending therebetween and separated by seam edges; and a second bowl-shaped casing shell having a second annular diffuser portion concentric with the first annular diffuser portion, a second annular downstream edge co-axial with the second diffuser portions and a smooth surface of revolution extending therebetween. The first and second bowl-shaped casing shells are concentrically nested. The second shell closes the grooves at the surface of revolution thus defining a diffuser at the first and second diffuser portions and a plurality of individual diffuser pipes extending from the diffuser to the first and second downstream edges when the seam edges of the first shell are secured to the surface of revolution of the second shell.
The first shell could be an inner shell, the surface having the grooves being an external surface thereof, and the second shell is correspondingly an outer shell the surface of revolution being an internal surface thereof; or vice versa.
Preferably the seam edges are located on lands extending laterally between adjacent grooves and the lands extending continuously the length of the grooves. This construction reinforces the structure to resist vibration through the diaphragm action of the lands which are preferably brazed to the surface of revolution of the second shell. The shells can be easily manufactured from metal, the first shell, for example, from castings and the second shell from sheet metal preferably in a pressing process, thereby eliminating much of the cost and time involved in fabricating prior art diffusers constructed of multiple bent pipes brazed to a separately machined hub.
Several significant advantages result from this novel diffuser design. The costs of production are reduced since tooling costs and manufacturing complexity are dramatically reduced when only two shell parts are required. Conventional diffuser assemblies in contrast, require the separate manufacture of several individual diffuser pipes, the machining of a diffuser hub and precise fitting and brazing of the pipes to the hub. Better performance results from elimination of the internal transversal steps which are present in prior art diffusers at the joint between the hub and each of the pipes. It is noted that the costs of production are further reduced in contrast to the diffuser assembly formed by the nested shells, each having opposing mating grooves, as described in Brand""s diffuser assembly. One of the grooved shells is replaced by a cover shell having a smooth surface of revolution which is easier and less expensive to manufacture, for example, using a sheet metal pressing process. Furthermore, the mating of the opposing grooves on each shell is replaced by securing the seam edges between the grooves on the casing shell to the surface of revolution of the cover shell so that the manufacturing complexity is further reduced.
The designer is freed from many of the constraints imposed by conventional diffuser manufacturing techniques. To a large extent, conventional diffuser configurations are dictated by the limitations of fabrication. Many trade-offs between diffuser performance and manufacturing costs compromise the efficiency of prior art diffusers.
The invention however, releases the designer from many of the considerations dictated by prior art manufacturing methods. Using the nested shells of the invention, the shape and cross-section of diffuser ducts become completely independent of the manufacturing method used, permitting the diffuser duct shape to be optimized for aerodynamic and structural efficiency.
By adoption of curved or variable diffusion diffuser ducts, the invention can result in lower overall engine weight by reducing the gas generator case diameter. In conventional engines, the diameter of the compressor impeller combined with the outwardly disposed diffuser assembly largely determines the gas generator case diameter. Any reduction in the outward diameter of the diffuser assembly will reduce the gas generator case diameter and lead to a smaller engine of lesser weight and reduced external drag. The invention provides the designer with the freedom to reduce the external diffuser diameter by curving the diffuser ducts inwardly or by using variable cross-sectional profiles for the diffuser ducts. It is also possible to integrate either the casing shell or cover shell, whichever is an outer shell into the casing wall of the gas generator to further reduce the overall engine weight.
The thickness of diffuser duct walls can be optimized for improved performance and minimum weight. If needed, reinforcement can be positioned in selected zones of increased thickness or may include external reinforcing ribs to control vibration, accommodate localized stresses or resist wear.
Design changes can be incorporated with considerably shorter lead time and development of new engines can proceed more rapidly. No tooling is needed to produce prototype testings. Solid model data can be used with laser photolithographic metal powder casting techniques to rapidly produce metal prototypes for example.
Further details of the invention and its advantages will be apparent from the detailed description and the drawings included below.